Subj : Re: object serialization
To   : netscape.public.mozilla.jseng
From : Daniel Fournier <zdev@noos.fr>
Date : Fri Mar 26 2004 07:43 am

Jens Thiele wrote:
> a first hack to get serialization working in "pure" javascript
> - intended for "pure" javascript objects)
> - keeping the prototype (__proto__ field) right
> - keep graph relationships
> the hash function is shit - is there any way to get a hashvalue for
> an object (perhaps based on its pointer/reference)?
> 
> karme
> 
> 
> // a first object serializer hack
> // based on code from Daniel Fournier  ( thanks!)
> 
> // todo this is shit
> function hashfunc(obj,key,depth) {
>   /*  if (!key) key='';
>   if (!depth) depth=0;
>   var i,t;
>   for (i in obj) {
>     key+=i;
>     t=typeof(obj[i]);
>     key+=t.substring(0,1);
>     if (t == 'string') key+=obj[i].substring(0,3);
>     if (t == 'number') key+=obj[i];
>     else if ((depth<3)&&(t == 'object')) key+=hashfunc(obj[i],key,depth+1);
>     if (key.length>60) return key;
>   }
>   return key;*/
>   return obj.toSource();
> }
> 
> function emptyproto(p) {
>   for (a in p) return false;
>   return true;
> }
> 
> function Serializer() {};
> 
> // enter new scope
> Serializer.prototype._push=function(v){
>   this.scope.push(v);
>   this.fqscope+=v;
> }
> // leave scope
> Serializer.prototype._pop=function(){
>   this.scope.pop();
>   var r='';
>   for (var i=0;i<this.scope.length;++i) {
>     r+=this.scope[i];
>   }
>   this.fqscope=r;
> }
> 
> // this object has been serialized
> Serializer.prototype._serialized=function(obj){
>   var key=hashfunc(obj);
>   var slot=this._hash[key];
>   if (!slot) slot=this._hash[key]=[];
>   slot.push({o:obj,s:this.fqscope});
> }
> // is this object already serialized?
> Serializer.prototype._getSerialized=function(obj){
>   var key=hashfunc(obj);
>   var slot=this._hash[key];
>   if (!slot) return false;
>   for (var i=0;i<slot.length;++i) {
>     if (slot[i].o==obj) {
>       return slot[i].s;
>     }
>   }
>   return false;
> }
> 
> Serializer.prototype.serialize=function(object, id){
>   if (!object) throw "object required";
>   if (!id) throw "id required";
>   this.scope=[];
>   this.fqscope='';
>   this._push(id);
>   this.append="";
>   this._hash=[];
>   var res=id + "=" + this._serialize(object)+";\n"+this.append;
> 
>   // debug hashfunc
>   /*  var key,slot,c,ctotal=0;
>   for (key in this._hash) {
>     c=-1;
>     for (slot in this._hash[key]) c++;
>     ctotal+=c;
>     print ("key: "+key+" collisions:"+c);
>   }
>   print ("total collisions:"+ctotal);
>   */
>   return res;
> }
> 
> Serializer.prototype._serialize=function(object){
>   var ot=typeof(object);
>   if (ot == 'string') {
>     this._pop();
>     return "'" + object + "'";
>   }
>   if (ot == 'function') {
>     this._serialized(object);
>     this._pop();
>     return object.toString();
>   }
>   if (ot != 'object') {
>     // numbers, booleans
>     this._pop();
>     return object;
>   }
> 
>   // i don't need to serialize dates
>   // if (object instanceof Date) {
>   //     this._serialized(object);
>   //     this._pop();
>   //     return object.toSource();
>   //     }
> 
>   this._serialized(object);
> 
>   var closeSymbol, objectString = '', stored = '';
>   if (object instanceof Array) {
>     // array
>     closeSymbol = ']';
>     objectString += '[';
>     var c=0;
>     for (var k in object) {
>       if ((stored=this._getSerialized(object[k])))
> 	this.append+=this.fqscope+"["+k+"]="+stored+";\n";
>       else{
> 	if (c>0) objectString+=',';
> 	c++;
> 	this._push("["+k+"]");
> 	objectString += this._serialize(object[k]);
>       }
>     }
>   } else {
>     // object
>     closeSymbol = '}';
>     objectString += '{';
>     var c=0;
>     for (var k in object) {
>       // test if this member comes from our prototype
>       if (object.__proto__[k]!=object[k]) {
> 	if ((stored=this._getSerialized(object[k])))
> 	  this.append+=this.fqscope+"."+k+"="+stored+";\n";
> 	else{
> 	  if (c>0) objectString+=',';
> 	  c++;
> 	  this._push("."+k);
> 	  objectString += k + ':' + this._serialize(object[k]);
> 	}
>       }
>     }
>     // serialize prototype
>     if ((object.__proto__)&&(!emptyproto(object.__proto__))) {
>       if ((stored=this._getSerialized(object.__proto__)))
> 	this.append+=this.fqscope+".__proto__="+stored+";\n";
>       else{
> 	if (c>0) objectString+=',';
> 	c++;
> 	this._push(".__proto__");
> 	objectString += "__proto__" + ':' + this._serialize(object.__proto__);
>       }
>     }
>   }
>   objectString += closeSymbol;
>   this._pop();
>   return objectString;
> }
> 
> // graph test
> foo={a:4};
> graph={x:foo, y:foo};
> eval(new Serializer().serialize(graph, 'graph2'));
> graph2.x.a=77;
> // should print 77
> print((graph2.y.a==77) ? "Test passed" : "Test failed");
> 

Hi Jens,

Sorry, but I have not enough time today to look carefuly at your code.

However, about the hash function, I can help:
There is a MD5 hashing class within Mozlib library, part of my JS 
Console extension (<http://jsconsole.mozdev.org>). Find it in the 
"chrome://mozlib/content/lib/js/digest/" directory (after installing JS 
Console, of course). You need only 2 files: generator.js and md5.js.

You could use the DIGEST_MD5Generator class to hash your objects, based 
on a unique field value, for instance using the Date function. For instance:

var object = {id:new Date}; //add your properties as you wish
var hashvalue = (new DIGEST_MD5Generator()).getHex(object.id);

Will it fit?

Daniel

.